The overall objective of this project is to study the genetic relationships of Corynebacterium diphtheriae and other aerobic corynebacteria to each other and the genetic mechanisms influencing that relationship. DNA-DNA homologies between C. diphtheriae and other aerobic corynebacteria will be determined with special emphasis on strains of Corynebacterium isolated from the skin. A survey will be undertaken of the plasmids in the various species of Corynebacterium included in the homology studies. The relationship between their plasmids will be examined by electron microscope observations, restriction endonuclease digest patterns as determined by agarose gel electrophoresis, and DNA-DNA homology studies of whole plasmids and restriction enzyme fragments. The mechanisms by which plasmids enter or are exchanged in corynebacteria as for example by conjugation, transduction or transformation will be investigated. The extent to which the gene for diphtheria toxin is carried cryptically in non-toxigenic C. diphtheriae and other corynebacteria will also be determined using a radiolabeled DNA fragment bearing the gene for toxin as the probe. The DNA fragment can be isolated from restriction digests of beta converting bacteriophage. Finally a comparative study of converting and non-converting corynephages will be continued as a means of examining the evolution of phage conversion in C.diphtheriae and the possible role of phages as transducers of plasmid or chromosomal DNA.